Radical-curing water-soluble polymers can be used without any organic solvent and are therefore favorable from the standpoint of economy, safety, and environmental hygiene. They have hence been extending their use in a broad range of applications as binders, sizes for textile, immobilizing agents for enzymes, and the like. Inter alia, water-developable image-forming materials have acquired importance in the field of printing, information processing, and electronics as, for example, masks for screen printing, resin letterpresses, and etching resists.
Polyvinyl alcohol (hereinafter abbreviated as PVA) has hitherto been used widely as an image-forming material in the form of a photo-curing system, such as a PVA-bichromate system, a PVA-diazonium salt system, because it has excellent film-forming properties to provide a water-resistant cured film. However, since these PVA photo-curing systems are poor in preservation stability and, in particular, the former system causes a toxicity problem arising from a chromium ion, various attempts have been made to introduce a photosensitive group into PVA itself to provide a photosensitive image-forming PVA derivative.
Techniques for modifying PVA are roughly classified into methods of introducing a photo-dimerizable photosensitive group and methods of introducing a radical polymerizable double bond (vinyl group) as a side chain. The latter group of methods are advantageous in that the radical polymerization has no theoretical upper limit in photon yield. Therefore, high sensitivity can be obtained and physical properties of a cured film, based on the ultimate use, can be selected by combining various other monomers. As a result, many processes for the latter method have been proposed.
However, most of the processes proposed for the introduction of a double bond into PVA involve a reaction step in an organic solvent and are therefore not economical. For example, JP-A-49-4738 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process in which PVA is reacted with an acid anhydride in pyridine or dioxane to isolate a purified carboxyl-modified PVA, and the modified PVA is dissolved in a solvent, e.g., dimethylformamide, and reacted with a glycidyl-containing unsaturated compound to introduce a double bond. Further, JP-B-48-6962 (the term "JP-B" as used herein means an "examined published Japanese patent application") discloses a reaction between PVA and maleic anhydride in an organic solvent (dimethyl sulfoxide or dimethylacetamide is used in the working examples). According to this process, however, maleic anhydride should be introduced in a large proportion in view of its low polymerization activity, and the resulting modified PVA is no longer water-soluble.
Introduction of a double bond into PVA in water has been proposed in JP-A-54-138090 and JP-B-49-5923. The former process comprises reacting PVA with acrolein or crotonaldehyde to form an acetal. The double bond thus introduced has poor reactivity. On the other hand, the latter process comprises reacting PVA with N-methylolacrylamide in water. This process attains a low reaction rate in practice, and fails to obtain modified PVA having sufficient photosensitivity.
JP-A-58-11931 proposes a process for introducing a (meth)acryloyl group into PVA which comprises reacting a completely or partially saponified general PVA with a compound having an epoxy group and a (meth)acryloyl group. According to the process, general PVA possesses one carboxyl group, at the most, per high molecular weight polymer molecule due to chain transfer of an acetyl group formed during polymerization of vinyl acetate, and the carboxyl group is reacted with an epoxy group to introduce a (meth)acryloyl group and, at the same time, the hydroxyl group of PVA is also reacted with an epoxy group. However, since the reaction between the hydroxyl group and the epoxy group in an aqueous medium is slow and, also, the number of the terminal carboxyl group is one or less per high molecular chain, the amount of a vinyl group introduced is extremely small, and, therefore, fails to afford sufficient photosensitivity. For this reason, the process uses a separately prepared photopolymerizable monomer in combination.